1. Field of the Invention
The present invention relates to a sound image localization apparatus and is preferably applied to the case where a sound image reproduced with a headphone, for example, is localized at a given position.
2. Description of the Related Art
When an audio signal is supplied to a speaker and reproduced, a sound image is localized ahead of a listener. On the other hand, when the same audio signal is supplied to a headphone unit and reproduced, a sound image is localized within the listener's head, and thereby an extremely unnatural sound field is created.
In order to improve the unnatural localization of a sound image in a headphone unit, there has been proposed a headphone unit adapted to enable, by measuring or calculating impulse responses from a given speaker position to both ears of a listener and by reproducing audio signals with the impulse responses convoluted therein with the use of a digital filter or the like, realization of localization of a natural sound image outside the head as if the audio signals were reproduced from a real speaker (see Japanese Patent Laid-Open No. 2000-227350, for example).
FIG. 1 shows the configuration of a headphone unit 100 for localizing a sound image of an audio signal of one channel outside the head. The headphone unit 100 digitally converts an analog audio signal SA of one channel inputted via an input terminal 1 by an analog/digital conversion circuit 2 to generate a digital audio signal SD, and supplies it to digital processing circuits 3L and 3R. The digital processing circuits 3L and 3R performs signal processing for localization outside the head, on the digital audio signal SD.
As shown in FIG. 2, when a sound source SP at which a sound image is to be localized is located in front of a listener M, a sound outputted from the sound source SP reaches the left and right ears of the listener M via paths with transfer functions HL and HR. The impulse responses of the left and right channels with the transfer functions HL and HR converted to time axes are measured or calculated in advance.
The digital processing circuits 3L and 3R convolute the above-described left-channel and right-channel impulse responses in the digital audio signal SD, respectively, and outputs the obtained signals as digital audio signals SDL and SDR. The digital processing circuits 3L and 3R are configured by a finite impulse response (FIR) filter as shown in FIG. 3.
Digital/analog conversion circuits 4L and 4R analogously convert the digital audio signals SDL and SDR to generate analog audio signals SAL and SAR, respectively, amplify the analog audio signals with corresponding amplifiers 5L and 5R and supply them to a headphone 6. Acoustic units (electric/acoustic conversion devices) 6L and 6R of the headphone 6 convert the analog audio signals SAL and SAR to sounds, respectively, and output the sounds.
Accordingly, the left and right reproduced sounds outputted from the headphone 6 are equal to the sounds which have reached from a sound source SP shown in FIG. 2 via the paths with the transfer functions HL and HR. Thereby, when the listener equipped with the headphone 6 listens to the reproduced sounds, the sound image is localized at the position of the sound source SP shown in FIG. 2 (namely, outside the head).